We have found that under certain conditions late in the growth of a culture of E. coli, the lysine-sensitive aspartokinase (AKIII) activity drops rapidly. These conditions include optimal oxygenation, a temperature above 30 degrees, and growth of glucose as carbon source. The physical and catalytic properties on the active enzyme remaining are different from the enzyme obtained early in the culture. We have reason to think that adenylylation or some similar process may be involved and are following this lead to try to find out in what form P32 is incorporated into the enzyme, and what relationship this phenomenon bears to the loss in molecular weight, decrease in substrate cooperativity and splitting of the enzyme into two large fragments which cross react immunologically with the native enzyme. By depriving the culture of oxygen, by downshifting the temperature to 23 degrees or by adding metabolic inhibitors we hope to isolate intermediates in the degradation process. The phenomenon we are studying appears to be a proteolytic digestion of AKIII triggered by some other alteration of the molecule, which in turn requires either oxidative metabolism or an active citric acid cycle.